1. Field of the Invention
The present invention relates to a resin composition of saponified ethylene-vinyl ester copolymer (hereinafter, called as “EVOH resin”) having gas barrier property against long-time hot water treatment, and more particularly to a resin composition that enables to suppress elution of EVOH resin against retort treatment, and produce a packaging film having excellent gas barrier property and long-run processability, as well as a multilayer structure using the resin composition.
2. Prior Art
Saponified ethylene-vinyl ester copolymer (EVOH resin) is utilized as a food packaging film and the like, because of its excellent gas barrier property against oxygen, odor, and the like. A film made of EVOH resin, however, may be whitened, or the gas barrier property of the film may be lowered, or a package made of the film may be deformed, if the film is subjected to long-time hot water treatment such as retort treatment.
A resin composition comprising EVOH resin and a drying agent is known as a resin composition capable of producing a packaging film having excellent gas barrier property against hot water treatment. For instance, U.S. Pat. No. 4,792,484 proposes a multilayer film produced by sandwiching an intermediate layer of a resin composition containing dispersed phosphate salt particles such as sodium phosphate, as a drying agent, between thermoplastic resin films of polyethylene, polypropylene, polyvinyl chloride, or the like. The multilayer film is capable of preventing the gas barrier property of EVOH resin from lowering by water absorption, utilizing that the drying agent absorbs moisture intruded through an end of the film, or moisture permeated through an outer layer of the thermoplastic resin film.
There is also known a technique of adding polyamide resin to impart hot water resistance to EVOH resin. For instance, JP2001-151972A discloses that a multilayer film using a resin composition containing 70 to 95 wt % of EVOH resin, 3 to 25 wt % of aliphatic polyamide resin, and 2 to 13 wt % of semi-aromatic polyamide retains the gas barrier property, even after being subjected to a condition of 20° C. and 65% RH for three weeks; and that the shape of a packaging film using the multilayer film is retained even after heat sterilization, with no or less whitening.
The applicant has filed an application (see JP2000-136281A) directed to a multilayer film for use in boil sterilization or retort sterilization, comprising an intermediate layer of a resin composition containing EVOH resin, alcohol-soluble polyamide resin, and phosphate salt; and outer layers of a thermoplastic resin film. The publication discloses that the resin composition of the intermediate layer has excellent long-run processability, gas barrier property against retort treatment, and appearance (no or less whitening). The resin composition disclosed in the publication is prepared by: immersing a porous precipitate of EVOH resin in an aqueous solution of phosphoric acid to introduce the phosphoric acid in the EVOH resin, followed by drying in a batch type fluid bed dryer and then a batch type hopper dryer; and melt-kneading the polyamide resin.